Fundamental- distortion in Signal measured from a Signal generator through Oscilloscope

Thread Starter

blackite

Joined Oct 6, 2021
6
I have a doubt whcih is very fundamental. i wanted to generate a 100mv sinus signal from a signal generator and measure it on an oscilloscope. Actually to do this i attached a two sided BNC cable. The frequency was 100Khz.
1) First measurement- One side to signal generator and the other side to oscilloscope.
2) A second measurements i did was used one of the oscilloscope probe and directly attached the probe to BNC input port in the signal generator. Please see the picture attached to this thread.
As shown in picture the the oscilloscope signal - blue signal is from the first measurement BNC to BNC and the second measurement is the yellow signal. My question why are there distortions in the second measurement. I assume they are reflections whne i measure directly form the probe and in 2nd measurement the oscilloscope is filtering? But according to my knowledge the oscilloscope should show me the complete bandwidth also when a bnc cable is attached. I want to understand what exactly the oscilloscope is doing here because i want to use the sine wave generated form the SG for one of my projects.
 

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hexreader

Joined Apr 16, 2011
540
Not sure where all of that noise is coming from. Maybe correct termination would help, but maybe not...

Termination is important (probably 50 Ohms)

Dave explains termination well here:
Not necessarily relavant to your problem, but hopefully educational anyway.
 
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Thread Starter

blackite

Joined Oct 6, 2021
6
I saw the video and it at time 6:07 the person plugs a probe into signal generator and he get a very clean signal. In my second measurement i did the same just instead of using a probe to BNC adapter i just inserted the probe into the oscilloscope. But my oscilloscope shows a distorted signal. Ofcourse the 1st measurement is a clean signal (BNC to BNC) probably because SG and OSCI both matche their impedabnce. Why is the probe measurement showing distortions? Hope its clear what i mean.
 

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hexreader

Joined Apr 16, 2011
540
Apologies - my understanding is back-to-front. I mistakenly read that BNC signal was bad :(
Dave has another video (which may or may not be relevant) on noise pick-up from flying ground connector on oscilloscope probe.
Will search for that video and edit this post with a link when I find it.

EDIT: I just had a better idea.... Post this question on EEVBLOG test equipment sub-forum. I predict that you will get faster and better responses.
... and watch all of Dave's Oscilloscope videos. They are very good.
 
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Thread Starter

blackite

Joined Oct 6, 2021
6
Apologies - my understanding is back-to-front. I mistakenly read that BNC signal was bad :(
Dave has another video (which may or may not be relevant) on noise pick-up from flying ground connector on oscilloscope probe.
Will search for that video and edit this post with a link when I find it.

EDIT: I just had a better idea.... Post this question on EEVBLOG test equipment sub-forum. I predict that you will get faster and better responses.
... and watch all of Dave's Oscilloscope videos. They are very good.
I did post it in EEVblog. Would update here if i find a solution. Thanks
 

Ian0

Joined Aug 7, 2020
3,785
Turn the timebase right down to 5ms. You should see your 100kHz signal as a wide band. Does the band have 50Hz wavy edges?
 

Thread Starter

blackite

Joined Oct 6, 2021
6
Turn the timebase right down to 5ms. You should see your 100kHz signal as a wide band. Does the band have 50Hz wavy edges?
If this is what you mean.the yellow one is distorted and blue the clean measurement. please see picture -1
Also frequency spectrum shows some unwanted frequencies between 400Khz and 500 Khz. pls see picture 2
 

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sparky 1

Joined Nov 3, 2018
614
A summary of putting together precision test equipment to do a distortion analysis.
( An iphone app might be free, does good job for audio testing.

However the sweep of the output may not be enough.
When a microphone samples a room it can identify how a music system is affected by the room acoustics.
A video shows one approach on improving room acoustics.
(By comparing calibration source with an adjusted source to counter front back issues. )
 
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BobaMosfet

Joined Jul 1, 2009
1,900
I'm also wondering if the
You say there is distortion but your yellow and blue signals have no distortion. Instead there is a lot of wideband random noise and we do not know what is producing it.
Has TP verified the quality of their probes..., or made sure they are calibrated, etc? Is the scope calibrated? Just cover all bases.
 

Thread Starter

blackite

Joined Oct 6, 2021
6
I found the solution while further testing for faulty probes. The noise is measured by the Oscilloscope because my bandwidth was set to 1GHz. To test this i measured the voltage output of a 9v battery and observed that it was showing oscillating noisy signal between 8.9v and 9.3v which according to me should not be the case. it should be quite precise. Reducing the bandwidth to 20 Mhz did improve the battery signal shown on Oscilloscope and the SG's Sinus signal shown on Oscilloscope.
thank you everyone for your recommendations and suggestions. I learned something new that depending on the frequency of the input signal from signal generator i should have a suitable bandwidth set in my oscilloscope otherwise the probe will measure unwanted noise signals.
 
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